Material showing on wrong faces
You need more geometry. The subsurf modifier works by interpolation, meaning that is subdividing the geometry, the boundaries for materials are also interpolated. On the sides of the guitar you need to add supporting loops, those will help you define the edges for the material as well.

As an example of the issue, this cylinder has two materials, one for the top and bottom faces, and one for the sides. The subsurf modifier interpolates mapping of the materials clearly. Adding supporting loops will fis the mapping error: